Groups: policy, group-JID allowlist, and that participants are still authorised by
the gateway sender allowlist or pairing (`open` alone admits nobody without one);
`require_mention` defaults to false; WHATSAPP_GROUP_POLICY / WHATSAPP_GROUP_ALLOWED_USERS
rows in the environment reference. The alt-id node tests collapse to one (the
participantAlt case duplicated the first-contact case; the "still resolves via mapping
files" case only re-asserted matchesAllowedUser).
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